1. Field of the Invention
The present invention relates to a light guide plate for liquid crystal displays and a surface lighting device incorporating the light guide plate, and more particularly to a light guide plate having a large size display surface as well as uniform and intensified luminance over the entire surface.
2. Description of Prior Art
Surface lighting devices are capable of emitting light uniformly over a wide area, and are thus suitably applicable for various light sources. In particular, a liquid crystal panel in a liquid crystal display (LCD) does not itself emit any light. Usually, the liquid crystal panel is, illuminated from its rear side using a light source. This improves display conditions and quality, for easier observation of images displayed on the LCD.
Light emitting from a surface lighting device serving as a backlight source is required to have a luminance that is uniform and strong over an entire light emission surface. In addition, power consumption should be low and lighting efficiency high, and the surface lighting device should be small and lightweight.
Referring to FIG. 3, a conventional backlight module 1 includes a light source 11, a wedge-shaped light guide plate 12, a reflection board 13, a diffusion board 14 and two prisms 15. In addition, the light source 11 is covered with a lamp shield 111 for reflecting light back into the light guide plate 12, so as to improve a lighting efficiency of the light source 11. Incident rays of light emitting from the light source 11 transmit directly through an incident side (not labeled) into the light guide plate 12, or are reflected back into the incident side of the light guide plate 12 by the lamp shield 111. The light rays are uniformed by the light guide plate 12 and the diffusion board 14, and then emit from the backlight module 1. However, the backlight module 1 has a high luminance in a vicinity of the light source 11, and a low luminance far from the light source 11. If the light guide plate 12 has a particularly large surface area, a difference between the low luminance and the high luminance is very significant. Therefore, this kind of light guide plate is not suitable for a large-sized liquid crystal display.
Referring to FIG. 4, U.S. Pat. No. 5,438,484 discloses an improved large-sized surface lighting device 2 and its application in an LCD. The surface lighting device 2 comprises a light guide plate 22, which is divided into two light guide portions 221, 222 at a center thereof. The light guide portions 221, 222 are bonded by a transparent adhesive, thereby forming a junction 23. A pair of light sources 211, 212 is arranged adjacent two lateral incident sides 241, 242 of the light guide portions 221, 222 respectively. Incident rays of light emitting from the light sources 211, 212 transmit directly into the light guide portions 221, 222 through the incident sides 241, 242 respectively, and then emit from adjacent light emission surfaces 25 of the light guide plate 22. Some of the rays of light are reflected back by the junction 23, so that a luminance in a vicinity of the central portion of the light guide plate 22 at the junction 23 is intensified. Thus, relatively high luminance exists both at the central portion of the light guide plate 22 and at portions of the light guide plate 22 in the vicinities of the incident sides 241, 242 respectively. Yet the intensity of high luminance at the central portion of the light guide plate 22 is generally different from the intensities of high luminance at the portions of the light guide plate 22 in the vicinities of the incident sides 241, 242. In addition, relatively low luminance exists in intermediate portions of the light guide plate 22 that are between the central portion and said portions at the incident sides 241, 242 respectively. That is, differences between the high luminance portions and the low luminance portions are still a problem. If the light guide plate 12 has a particularly large surface area, the differences between the low luminance and the high luminance are very significant. Moreover, an overall luminance of the entire display surface is necessarily reduced.